Magnetic media today extensively use magnetic particles dispersed in a non-magnetic binder as the coating for discs and tapes. Gamma ferric oxide is typical of the magnetic particles used and derives its magnetic properties largely from shape anisotropy rather than magnetrocrystalline anisotropy. Thus the particles are acicular (needle-like or rod-like) with high aspect ratios as shown in the table of properties below:
______________________________________ Coercivity 300-500 oersteds Saturation Magnetization 70-75 emu/gramme Remanence 25-30 emu/gramme Particle Size 400-1000 nanometers Surface Area (BET) 15-30 m.sup.2 /gramme Aspect Ratio 6:1 to 10:1 ______________________________________
The particles have a single preferred or "easy" axis of magnetization in the longitudinal direction and it is common practice to orient the dispersed particles during coating to maximize the recording properties of the tape or disc.
As shown by Pingand in U.S. Pat. Nos. 3,843,773 and 4,376,714 and by Audran et al in U.S. Pat. No. 3,928,709, acicular iron oxide particles are obtained by the conversion of acicular non-magnetic, alpha ferric oxide hydrate particles (.alpha.-FeOOH) to acicular, magnetic, ferrous ferric oxide (Fe.sub.3 O.sub.4) by dehydration and reduction in an atmosphere of hydrogen or natural gas at temperatures between 350.degree. C. and 500.degree. C. The magnetite produced is then oxidized to acicular, magnetic gamma ferric oxide (.gamma.-Fe.sub.2 O.sub.3) particles by heating in air at 250.degree. to 300.degree. C.
The coercivity range obtained with gamma-ferric oxide powders is limited whereas for several applications e.g., videotapes, digital recording, higher coercivity may be demanded. The introduction of cobaltous ions into the crystal lattice of the oxide significantly enhances the magnetic properties, providing coercivities ranging from 400 to 1000 oersteds, due to increased magneto-crystalline anisotropy. However uniformly-modified gamma iron oxide particles are sensitive to both thermal and mechanical stress as is discussed in "The Complete Handbook of Magnetic Recording" by Finn Jorgensen, pages 192-193, for example. On the other hand surface-modified or epitaxially-modified gamma iron oxide particles, wherein the cobalt ions may be present primarily in a thin layer at the surface of the particle, are significantly more stable than the uniformly-modified particles yet retain the enhanced magnetic properties. These properties and the superior thermal stability of the surface-modified, acicular, oxide particles are illustrated in data presented by A. Eiling et al in IEEE Transactions on Magnetics, Vol. MAG-22, No. 5, pps. 741-743 (1986).
As another approach to realize increased coercivity with oxide particles and therefore increased recording densities in magnetic media, mixtures of gamma iron oxide and magnetite particles have been considered. Thin films containing mixtures of gamma iron oxide and magnetite crystals were shown by N. Borelli et al, IEEE Transactions on Magnetics, Vol. MAG-8, 648(1972) to exhibit high coercivities. In an article entitled "High Coercivity Particulate Magnetic Media via In-Situ Partial Reduction of gamma Fe.sub.2 O.sub.3 and Modified gamma-Fe.sub.2 O.sub.3 " by V. M. DePalma, M. F. Doerner and A. M. Ward, published in IEEE Transactions on Magnetics, Vol. MAG-18, No. 6, 1083 (1982) there is described a process for increasing the coercivity of particulate media involving an anaerobic cure followed by annealing in air at low temperatures, typically 60.degree. C. to 95.degree. C. for 100-1000 hours.
Interest in multi-axial or cubic or isotropic particles has been maintained within the recording industry. Such particles offer the capability of producing media with high coercivity and, in addition, could allow significantly higher ratios of pigment to binder to be used in the coatings. Furthermore, an isotropic particle offers the opportunity to record in both the longitudinal and perpendicular directions. It is therefore extremely desirable to develop stable, isotropic, highly magnetic, ultrafine particles.
A method of producing ultrafine particles of magnetite (Fe.sub.3 O.sub.4) is described in U.K. Pat. No. 1,142,214 and the particles are claimed to be "substantially isotropic". The method involves control of both the pH and the temperature of a suspension of oxide-hydroxide particles while sparging with an oxidizing gas (air) to give ultrafine particles of magnetite. The particles produced by this method typically have an average diameter of 100 nanometers, with a distribution of particle sizes ranging from 50 to 1000 nanometers.
An electrochemical method for producing ultrafine, isotropic particles of magnetite or cobalt-modified magnetite is desribed by H. B. Beer et al in U.S. Pat. No. 4,474,653. Here careful control of the concentrations of ionic reactants and oxidant species at controlled pH leads to the formation of ultrafine particles. Particles of magnetite or cobalt-modified magnetite produced by this method are isotropic, with an average particle size of 20-40 nanometers and with a narrow particle size distribution of 15-150 nanometers as is shown in FIG. 1.
However the isotropic powders produced either as taught in GB Pat. No. 1,142,214 or U.S. Pat. No. 4,474,653 exhibit sensitivity to thermal and mechanical stress. For example the temperature coefficient of coercivity for a cobalt-modified isotropic ferrite powder is typically -1.0% per degree Celcius, i.e., for a powder of coercivity 800 oersteds, the loss of coercivity with increased temperature is 8 oersteds per degree.